LibreOffice uses its own toolkit called VCL which uses a number back-ends to render using native toolkits on all platforms, similar in concept to WxWidgets. I don’t know if the way VCL works allows Qt6 fractional scaling to just work, but a Qt6/KF6 back-end already exists and can be forced by setting SAL_USE_VCLPLUGIN=qt6 or SAL_USE_VCLPLUGIN=kf6, so you should be able to try it out right now.